Application of Robotics to the Assembly of Missile Seekers
One of Raytheon’s key manufacturing strengths is its production of missile systems. Raytheon manufactures missiles for air, land, sea and space applications, including interceptors for U.S. ballistic missile defense. A key part of these missiles is the seeker, a device used to sense a target and guide the missile to the target location. Optical and infrared missile seekers are typically comprised of detectors, optical elements, electronics and mechanical assemblies with production rates ranging from hundreds to thousands of units per year.
Historically, assembling a missile seeker has been a labor intensive process, dependent primarily on the skill of highly trained operators. As build quantities have increased, demand has grown for greater production capacity, which, in turn, has increased the need for additional operators. While some intricate and precise assemblies continue to require the efforts of specially trained operators, others are potential candidates for automation with the opportunity to reduce the demand on our skilled workforce. One form of automation, the robotic cell, is particularly effective for missile seeker type assembly operations. A robotic cell is a manufacturing cell that uses a robot to perform some, if not all, of the assembly task set for a particular component. The cell is capable of maneuvering to exacting tolerances with speed and repeatability. It is also programmable for application to several different parts or subassemblies.
Raytheon performed trade analyses across the missile seeker product line to evaluate candidate assemblies for production by robotic cells. The suitability of a component for this type of automation depends on such factors as build quantity, intermittent assembly schedules, design/automation process compatibility and the use of existing processes that are highly variable. The impacts of the current process on human factors such as ergonomics are also considered. For example, automation can eliminate tedious and repetitive tasks that are associated with repetitive motion injuries. Finally, estimated recurring cost savings from reduced assembly labor and reduced assembly defects are used to project an automation return on investment (ROI). Ideal candidates for robotic assembly are those that best meet the overall goals of reducing cycle time, assembly costs, work in process (WIP) inventory and process variation, while at the same time improving product quality and providing an acceptable ROI.
Two assemblies, each for an optical seeker component, were chosen for an initial implementation of the automated robotic cell. The first assembly to be robotically implemented was based on a new design, consisting of two optical lenses bonded into a mechanical housing (see Figure 1). As this was a new design, robotic cell capabilities could be considered and the product design optimized for the automated assembly process. The second assembly was an existing design, consisting of four optical lenses bonded into a mechanical housing. This assembly had been manufactured by hand for years, and it required tight tolerances approaching the limits of the robotic cell.
Raytheon developed a custom automation cell (see Figure 2) capable of fully assembling both types of seeker components and requiring minimal human attendance. Additionally, since the cell was purposely designed around common assembly process steps, it can later be applied to additional components. The automation cell utilizes a Fanuc six-axis robot with the ability to reach specific points and paths in a very accurate and repeatable way. Assembly work is performed through the use of “end effectors,” as shown in Figure 3. An end effector is a selectable tool that attaches to the end of the robotic arm to perform a specific task. Different end effectors are exchanged during the assembly cycle using an automatic robotic tool changer. The robotic tool changer provides a powerful common interface between the robot and multiple custom tools, and provides the following functions:
- A mechanical mount.
- Fluid connections for air, vacuum and liquids.
- Electrical connections for power, controls and communications.
- An end effector tool identification capability.
The tool changer is an affordable and mature device commonly used in the commercial automation industry.
Several robotic cell automated assembly capabilities were developed specifically for use with seeker components. These include pick-and-place, cleaning, vision inspection, adhesive dispensing and ultraviolet (UV) curing operations.
The robotic pick-and-place of the optical lenses is enabled through the use of a custom designed end effector, which incorporates a vacuum pick-up feature that avoids contact with the clear aperture of the lens (see Figure 4). This ensures the part stays clean and damage free. Vacuum tools may be interchanged depending upon the size and geometry of the optical elements being handled.
The automated cleaning of optical components prior to bonding is performed through the use of an atmospheric plasma cleaner. The plasma cleaner produces a high density plasma “plume” that effectively cleans, activates and maximizes adhesive wettability of the targeted component surface. The plasma treatment provides a superior surface for bonding and results in optimized bond strength and performance.
Three cameras comprise the vision systems of the automated robotic assembly cell. One camera is mounted on a custom end effector, which moves into position by way of a robot arm to examine stationary objects. The second and third stationary cameras examine the adhesive dispense needle for alignment calibration (see Figure 5). Collectively, these cameras:
- Ensure the correct parts are present for the assembly step and that the parts are properly oriented (face up or down, etc.).
- Confirm the kit is complete with all necessary parts.
- Ensure all necessary end effectors are properly staged for automated access.
- Provide dispense needle “calibration” (i.e., X, Y, Z axis offsets for dispense motion).
- Ensure proper alignment between parts to be bonded.
- Verify assembly materials are dispensed properly at the beginning and end of the process.
The application of adhesive onto critical bonding surfaces is automated through the use of another custom end effector (see Figure 6). The adhesive dispense process is regulated through the use of multiple process controls. These process controls include:
- Checking adhesive expiration date.
- Monitoring a thaw timer for frozen adhesives to ensure material readiness for application.
- Monitoring material pot life1to ensure proper dispense pressure during application.
- Dispense tip cleaning2.
The use of UV curable adhesives drastically reduces material curing times from hours to seconds. The robotic cell uses a custom end effector with a focused beam of UV light to apply UV energy to the bond line of the adhesive.
Efficient use of an automated cell supporting multiple products is dependent on the ability to rapidly change from one product to another with minimal down time. This product change-over is accomplished through the use of work cell software programming and parts presentation pallets (see Figure 7) which are designed with kinematic mounts to quickly and precisely align their location within the automated cell. In this robotic cell implementation, the changeover from one assembly to the other is accomplished in less than 5 minutes.
The advancement of the Raytheon automated robotic assembly cell has led to a more precise, repeatable, flexible and cost-effective process for assembling optical components. The initial implementation resulted in significant cycle time reductions and touch labor savings. It also provides a baseline automated assembly capability that can be tailored for future seeker and other Raytheon product manufacturing.
1 Pot life is the period of time after mixing during which an adhesive remains suitable for use.
2 After each dispense, the tip must be cleaned to remove residual material buildup and ensure
smooth or continuous material flow.
Chad Spalt, Contributors: David Stockero,
Bob Monier, Tolga Yazicioglu, Eric Huelsmann,
Jessica Overby, Charles Scott, Bob Munger
and Tony Vulcano
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